greybus: connection: replace custom error function with dev_err

Remove custom connection error function and replace it with dev_err.

The standard error function provides more information in the message
prefix (e.g. includes the interface id), has a well-known semantics
(e.g. does does not add newlines to messages), and is even somewhat
shorter to type.

Note that some uses of the custom function were already adding double
newlines due to the non-standard semantics.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>